The present invention relates generally to methane detectors, methods of detecting methane in a gas mixture, methane concentration measuring devices and methods of measuring methane concentration in a gas mixture.
A serious problem in the use of gas sensors based on gallium oxide (Ga.sub.2 O.sub.3) films is that, given the simultaneous presence of several gases, the sensor signal is composed of the superposition of all present gas components with the same characteristics, e.g., all present gas components have a reducing effect. This behavior, generally called cross-sensitivity, poses a serious obstacle to the effective operation of methane warning systems or methane measurement devices based on sputtered semiconducting Ga.sub.2 O.sub.3 layers.
For example, given the simultaneous presence of methane and alcohol vapor or, respectively, solvent vapors, the lack of selectivity of the sensor elements causes an ambiguity of the sensor signal, and thus to false alarms of the detector. Or, a methane concentration is measured that deviates significantly from the actual methane concentration. Since the sensitivity S of the Ga.sub.2 O.sub.3 sensors to the more reactive components, i.e. the alcohol vapors or, respectively, solvent vapors, is very high (typically about one order of magnitude higher than to the less reactive gases), an additional reducing gas component, e.g. methane, cannot be detected with sufficient sensitivity.
Up to now, several solutions have been proposed for improving the selectivity of methane gas sensors based on Ga.sub.2 O.sub.3.
In the German patent DE 41 39 721, an arrangement is provided for the detection of gases and a method is known for the selective detection of at least one gas contained in a gas mixture. For the selective detection of at least one gas contained in a gas mixture, the operating temperature of the gas sensor is varied. The time curve of a signal that depends on the electrical conductivity of the sensor layer is evaluated. For this purpose, the operating temperature of the gas sensor is increased in a leap, and the time curve of the sensor signal is registered. The evaluation ensues by means of a Fourier transformation. A disadvantage of this method is the relatively large expense for the evaluation by means of the Fourier transformation.
The company SATE of Salerno, Italy, uses a method in which the CH.sub.4 sensor is operated for a certain time at a first temperature and operated for a certain time at a second temperature. This process is repeated periodically. Using this method, it is possible to avoid false alarms of the methane detector. However, the presence of methane in a gas mixture containing alcohol or solvent vapors cannot be detected using this sensor.
A method for selective gas detection is known from the prior art Sears et al., Selective Thermally Cycled Gas Sensing Using Fast Fourier-Transform Techniques, Sensors and Actuators B, 2 (1990), pp. 283-289. A fast Fourier transformation (FFT) is used for the analysis of the data. This method has the disadvantage that a high computing expense is required for the evaluation.
A method for the analysis of gas mixtures is known from the prior art Wlodek, S. et al., Signal-Shape Analysis of a Thermally Cycled Tin-Oxide Gas Sensor, Sensors and Actuators B, 1991, pp. 63-68. The function of change of conductivity over time is simulated by several summed Gauss functions. The indices of the Gauss functions represent characteristic features for the gases respectively present in the gas mixture. A considerable computing expense is also required here for the evaluation.
A method for determining the concentration of a single gas using an SnO.sub.2 gas sensor is known from the prior art Sears, W., et al., Algorithms to improve the selectivity of thermally-cycled tin oxide gas sensors, Sensors and Actuators, 1989, pp. 333-349. For this purpose, the conductivity is measured dependent on sensor temperature and time. For different gases, the result different sensor signal curves dependent on the concentration of the gas. However, a disadvantage of this method is that it can be used only for individual gases, i.e. this method cannot be used for a gas mixture consisting for example of air, methane and alcohol vapors.
Accordingly, there is a need for improved methane detectors, methods of detecting methane, methane concentration measuring devices and methods of measuring methane concentration that can overcome the problems associated with disturbing or masking gases such as solvents and which further provide accurate detection and measurement without requiring expensive equipment such as the expensive microprocessors required to perform the Fourier transformations, fast Fourier transformations and summation of Gauss functions required by the above-described methods.